During manufacturing of printed products, measures are typically taken to ensure a certain level of printing quality. This is particularly true in the field of security printing where the quality standards that must be reached by the end-products, i.e. banknotes, security documents and the like, are very high. Quality inspection of printed products is conventionally limited to the optical inspection of the printed product. Such optical inspection can be performed as an off-line process, i.e. after the printed products have been processed in the printing press, or, more frequently, as an in-line process, i.e. on the printing press where the printing operation is carried out.
Optical inspection systems which are basically adapted to inspect printed products at large are already available on the market. These inspection systems typically work in the RGB domain based on the to be now designated as classic threshold-based inspection methods. Such inspection methods are for instance disclosed in U.S. Pat. No. 5,384,859 and U.S. Pat. No. 5,317,390. These publications disclose so-called iconic pixel-difference or threshold inspection methods, i.e. inspection methods which are based on the analysis of pixel density differences between sample images of the printed products and reference images. The threshold parameters are usually defined based on a comparison of several master images, whereby mean values or standard deviations are determined in local regions of the images and are attributed corresponding thresholds or tolerances. These values and tolerances are then compared with actual image values measured on sample images of the inspected material.
The above threshold inspection methods exhibit a certain number of disadvantages as described in detail hereinafter. These inspection methods may be adapted for inspection of security documents, but under certain conditions. Threshold-based inspection methods are not directly suited for the inspection of security documents, as security documents are printed using specific printing processes (such as intaglio printing for instance) which are not commonly used in commercial printing. The conventional threshold-based inspection methods must accordingly be adapted to the specific printed features of security documents.
According to the current state of the art, iconic threshold image processing techniques (as described in the above-mentioned U.S. Pat. No. 5,384,859 and U.S. Pat. No. 5,317,390) are normally used because of the high production rates. These methods however have the disadvantage that high, but nevertheless tolerable fluctuations during the production process can lead to detection of pseudo-errors in regions of the inspected images where an abrupt change of contrast is present. In order to prevent such pseudo-errors from occurring, the said regions which are characterized by abrupt changes of contrast are typically rendered insensitive to error detection (i.e. by attributing high tolerances to these regions) so that the inspection process can be stabilized. Error detection in the regions having abrupt changes of contrast is thus made almost impossible.
Other optical inspection methods are known in the art. European patents EP 0 730 959 and EP 0 985 531 for instance disclose inspection methods which are based on “elastic” models which take into account possible deformations of the printed substrates. Perceptive inspection methods which simulate in a rudimental way the perception of the human vision are also known from international application WO 2004/017034 and from German patent application DE 102 08 285. Statistical methods based on a statistical analysis of image patterns are also known in the art but have not shown a sufficiently satisfying performance.
The above optical inspection methods are by definition limited to inspection of the optical quality of the printed products, such as whether too much or too little ink has been applied onto the printed material, whether the density of the applied ink is acceptable, whether the spatial distribution of the applied ink is correct, etc. While these systems are adapted to detect such printing errors in a relatively efficient manner, the known inspection systems are however unable to perform an early detection of progressively-building printing errors. Such printing errors do not occur in an abrupt manner, but rather in a progressive and cumulative manner. These printing errors typically occur because of a gradual degradation or deviation of the behaviour of the printing press. As optical inspection systems inherently exhibit inspection tolerances, printing errors will only be detected after a certain period of time, when the tolerances of the optical inspection system are exceeded.
Experienced printing press operators may be capable of identifying degradation or deviation in the printing press behaviour which could lead to the occurrence of printing errors, for instance based on characteristic noises produced by the printing press. This ability is however highly dependent on the actual experience, know-how and attentiveness of the technical personnel operating the printing press. Furthermore, the ability to detect such changes in the printing press behaviour is intrinsically dependent on personnel fluctuations, such as staff reorganisation, departure or retirement of key personnel, etc. Moreover, as this technical expertise is human-based there is a high risk that this knowledge will be lost over time, the only available remedy consisting in securing storage in one form or another of the relevant technical knowledge and appropriate training of the technical personnel.